I have a dataset with
event=death
time (from medical examination until death/censoring)
dose (given at examination time)
Two groups are considered, a non-exposed group (dose=0), an exposed group
(dose between 5 and 60).
For some reason there is a theory of the dose increasing its effect over
time (however it was only given (and measured) once = at the time of
examination).
I tested a model:
coxph(Surv(time,dod)~dose + dose:time)
Previously I tested the model in SAS:
proc phreg data=test;
model time*dod(0)=dose dosetime /rl ties=efron;
dosetime=time*dose;
run;
Without the interaction terms I get the same results for the two models. By
including the interaction terms I do not. The model in R gives a negative
coefficient for the interaction term which is expected to be positive (and
is so in SAS). The LRTs are also completely different.
TWO QUESTIONS:
1) Is it reasonable to bring in an interaction term when dose is only
measured once?
2) If yes, can anyone give a hint on explaining the difference between the
models in R and SAS?
Thanx in advance,
marianne
_________________________________________________________________
Nyhet! Hotmail direkt i Mobilen! http://mobile.msn.com/
______________________________________________
[email protected] mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
